As technology for decreasing the outer diameter of a stepping motor, the inventors of the present invention previously developed a stepping motor constructed as shown in FIG. 15. Such motor has bobbins 4 around which coils 6A and 6B are wound, placed at opposite sides of a magnet 2, along the axis thereof, which magnet is fixed to a shaft 1. As a result, the outer diameter of the stepping motor can be decreased to a diameter less than a conventional stepping motor having coils arranged around the outer periphery of a magnet.
Sideways U-shaped yokes 8A and 10A, and 8B and 10B are fixed, respectively, to opposite ends of each of the bobbins 4 on the outer periphery of the magnet 2 in the state in which the side plates (corresponding to pole teeth) of the yokes face each other. The yoke 8A (8B), having a long pole tooth, is out of phase by 90.degree. from the yoke 10A (10B) having a short pole tooth. The pole teeth of the yoke, facing each other along the axis, are out of phase from each other by 45.degree.. By supplying an electric current to the coils 6A and 6B, while the direction of the electric current is alternately changed, a rotor formed of the magnet 2 and the shaft 1 is driven in steps of 45.degree..
In the above-described stepping motor, the base end portions of the yokes 8A and 10A, and 8B and 10B are fixed to the bobbins 4. Therefore, there is a limitation on the accuracy of positioning between the tips of the pole teeth. There is a deviation of a position at which the rotor is stopped, caused by a deviation of the pole teeth, occurs, and causing problems, for example, making it difficult to increase the rotor rotation accuracy.
Because the yokes 8A and 10A, and 8B and 10B are small and high precision is required for the assembly thereof because of the above-described reasons, close attention must be paid to an assembly operation. Productivity is low and costs are high.